Should virus culture be the “golden standard” in COVID-19 diagnostics?

written byJanne Koskinen

posted onFebruary 25, 2021

Although the whole concept of the “golden standard” is controversial, the term is still often used even in diagnostic guidelines. Controversy stems from the fact that very rarely a single diagnostic method is unambiguously both more sensitive and more specific than other methods. The concept becomes more and more complex given the different use indications for testing.

Based on scientific literature, which method is given the status of gold standard in COVID-19 testing should depend on the intended use of the diagnostic method. The criterion is different whether:

  • exposure is being demonstrated (particularly sensitive PCR test)
  • possible infectivity (PCR) is ruled out
  • or population is being screened for infectious individuals to prevent the spread of the virus and combat the pandemic (culture or sensitive antigen test).

According to international diagnostic regulations and quality system standards, diagnostic tests must be designed for a specific use and purpose. As discussed in Nature, COVID-19 antigen tests are designed to be appropriately sensitive to detect the majority of infectious cases, and in particular super-spreaders. Antigen tests do not respond to viral gene residues. Other test methods should be designed applying similar principle and their indication of use should reflect the test design.

It has been suggested that viral culture is not a sensitive enough method as the gold standard for COVID-19 testing. It is true that in method comparison studies, PCR finds more positives than viral culture, but are these additional findings important for future infection control (especially when the PCR result time is still usually 1-2 days)? The analysis becomes even more interesting when antigen tests results are added to the equation.

If a significant number of infectious individuals were missed by culture (due to the presence of infectious virus in the sample but the virus would not grow in culture for some reason), studies would have samples which have low PCR Ct value and high titre of virus (antigen test positive), but culture would still be negative. However, numerous scientific studies indicate that this does not appear to be the case (see, e.g., Pekosz et al. 2021, Figure 1A and Rusanen et al. 2020, Figure 2A). The results from these studies support the role of viral culture as the best reference for infectiousness. Sure, for routine diagnostics, culture is slow and laborious, but that’s a different question than what is the best reference method. The research evidence clearly shows that PCR positivity does not mean that the individual is contagious.

In addition, the usefulness and effectiveness of result time in pandemic management should be considered. Even if the central laboratory test (PCR or culture) was the most sensitive and specific, the effectiveness of testing in pandemic management is weak if the result is obtained with a delay. According to research evidence, the testing turn-around-time is more important than absolute sensitivity in pandemic control . How can we then justify that a slow test is the “golden standard” in COVID-19 diagnostics?

In the words of Dr. Michael Mina, Harvard epidemiologist “It might not catch every last one of the transmitting people, but it sure will catch the most transmissible people, including the superspreaders.” Scientific evidence now unequivocally shows that when comparing viral culture, antigen testing, and PCR to control an epidemic and test for infectious patients, rapid antigen testing is the most effective and accurate tool. PCR is not the most appropriate tool for assessing infectivity  because it reacts to small amounts  of RNA / DNA in samples due to persistence or contamination. Antigen testing is at least as accurate  as SARS-CoV-2 viral culture in assessing infectivity, and results are obtained while waiting. Antigen testing also works to identify pre-symptomatic  and asymptomatic infectious individuals. Can large-scale antigen testing coupled with efficient vaccination be the answer to finally end the pandemic?